Dew, Ten Grains of Wool, and the Night Sky

Read the episode (Narration script)

Welcome to Origins. In this program we explore the technological and scientific roots of the modern world. Let's begin.

On a clear autumn morning, two cars sit in the same driveway. One is parked in the open, and its windshield is white with frost. The other is under a carport, a few steps away, and its glass is dry. The same air touched both of them all night. So what did the open car have that the other one didn't?

It had a view of the sky. The person who worked out why that matters was a London doctor with a bag of wool, a friend's garden, and a habit of staying up very late.

For most of history, dew seemed to arrive from somewhere else. Writers argued over whether it fell from the heavens as a fine, invisible rain, or rose out of the warm ground. Aristotle had noticed one thing that held up: dew appears on calm, clear nights. Nobody could say why clear mattered.

William Charles Wells was born in Charleston, South Carolina, in 1757, the son of a printer and bookseller who sided with the Crown when the Revolution came. William trained in medicine at Edinburgh, where his doctoral thesis was on the subject of cold, and in 1784 he settled in London as a physician.

That autumn, a rough experiment gave him an idea: dew and cold seemed to arrive together. Others saw it too. James Six of Canterbury laid one thermometer in a meadow and hung another six feet above it, and on clear nights the one on the ground always read lower. The natural conclusion was that forming dew makes things cold.

Wells carried the question around for more than twenty-five years. In 1811, on a calm, clear night, he laid a thermometer on dewy grass and hung a second two feet above it. An hour later, the grass was eight degrees Fahrenheit colder.

Then he went to work in earnest, in a garden in Surrey about four miles south of Blackfriars Bridge, with a strip of mown grass sixty-two feet long. His main instrument was wool. He divided white wool into parcels of ten grains each, less than a gram, teased each into a flattened ball about two inches across, set them out in the evening, and weighed them in the morning. Whatever they gained was dew.

He raised a painted board four feet off the grass on thin legs, with one parcel of wool on top and another tied underneath. The two tufts sat an inch apart, in the same air. One night the top parcel gained fourteen grains and the bottom one four. Another night, twenty and four.

Then came the test of whether dew falls. He stood a hollow clay cylinder upright on the grass, open at the top, and put wool inside. On a perfectly still night, falling moisture would drop straight in. The wool inside gained barely two grains. Wool in the open gained sixteen. The cylinder blocked nothing directly overhead. It blocked most of the sky.

Next he turned the old idea around. One January day in London, he set dry wool on pasteboard over snow. In thirty-five minutes it was five degrees colder than the air, without gaining any weight. The cold came first, and the dew followed, the same way water beads on a glass of iced lemonade. A cold surface chills the air touching it until that air can no longer hold its water vapor.

The clouds confirmed it. One night the grass sat at thirty-two degrees. Clouds moved in, and twenty minutes later it was thirty-nine. The sky cleared, and twenty minutes after that it was back to thirty-two.

Here is the mechanism Wells assembled, building on an idea from the Genevan physicist Pierre Prevost. Every object constantly gives off heat as invisible radiation and constantly receives it from everything around it. Grass under a tree or a cloud gets a good share back. Grass facing a clear night sky is facing, in effect, deep space, which sends almost nothing back. Grass conducts heat poorly, so the soil's warmth climbs up slowly, and still air doesn't stir enough to rewarm it. The blade sinks below the temperature of the air, and water comes out of that air onto the blade. Wind spoils the effect by mixing in warmer air. Clouds spoil it by sending heat back down.

That explained something that had embarrassed him. Wells admitted he had smiled, "in the pride of half knowledge," at gardeners who threw thin mats over tender plants on cold nights. So he stretched a worn cambric handkerchief, thin and torn in places, over four sticks six inches above the grass. Air flowed freely underneath. Yet on a night when the open grass was eleven degrees colder than the air, the grass under the handkerchief was only three degrees colder. The gardeners had been right without needing the reason.

There was a stranger case from India. In 1775, Sir Robert Barker described ice-making at Allahabad to the Royal Society. Workers dug shallow pits about thirty feet square, lined them with dried sugarcane or corn stalks, and set out rows of shallow, unglazed clay pans of boiled water at dusk. Before sunrise they gathered the ice, enough to supply Barker's table through the summer. Clear, calm nights worked best, and wind or cloud could ruin a night's work. The water froze while the air stayed several degrees above freezing.

Wells dug his own pit in Surrey one May evening, laid down dry straw, and set out pans of boiled water. At one in the morning, with the air four feet up still above thirty-nine degrees, ice formed in small glasses of water on the grass. By two, there was ice in the pit. In an English May, he had made ice by pointing water at the sky.

The Essay on Dew appeared in 1814 and won the Royal Society's Rumford Medal. Wells died three years later. A posthumous volume included a short paper on human skin color that sketched selection by survival, and Charles Darwin later credited it as the first recognition of natural selection he knew of.

The cold sky Wells measured is still overhead. In 2014, engineers at Stanford put a thin layered panel on a rooftop that reflected sunlight and shed its heat as infrared light at wavelengths the atmosphere lets pass out to space. In full sun, it sat about five degrees Celsius below the surrounding air.

And the frosted car is doing what the Allahabad pans did: radiating heat all night to a clear sky that sends almost none back, while its neighbor trades heat with a roof. The explanation came from patient work in odd conditions: thermometers in the grass, wool on a board, a doctor in a dark garden writing down numbers until he could account for the morning.

This has been Origins. Thanks for listening.